Immunocytokines (antibody-cytokine fusion proteins) were first reported in the literature in the early 1990s and consisted of whole antibody fusions with cytokines such as lymphotoxin (TNF-α) or interleukin 2 (IL-2). Subsequent studies in GD2-expressing tumour models in mice indicated that the ch14.18 antibody and ch14.18-IL2 immunocytokine both had anti-tumour activity but that the immunocytokine was far more potent than the antibody, even when combined with free IL-2, (see Sabzevari H et al., Proc. Natl. Acad. Sci. USA, 1994, 91:9626-30; Pancook J D, et al., Cancer Immunol. Immunother., 1996, 42:88-92; Becker J C, et al., Proc. Natl. Acad. Sci. USA, 1996, 93:2702-7). In addition, immune-competent mice treated with the immunocytokine, but not the antibody plus IL-2, developed an adaptive immune response dependent on CD8+ T-cells that prevented subsequent tumour challenge (Becker J C, et al., J. Exp. Med., 1996, 183:2361-6; Becker J C, et al., Proc. Natl. Acad. Sci. USA, 1996, 93:7826-31). Thus, the targeting of IL-2 to the tumour microenvironment induces an anti-tumour vaccine effect that is not possible with the antibody, either alone or together with the free cytokine. A related humanized immunocytokine, hu 14.18-IL2, achieved clinical proof of concept in relapsed non-bulky neuroblastoma as monotherapy where it induced a significant number of complete responses in patients with no other treatment options (see Shusterman et al., Journal of Clinical Oncology, 2010, 28(33), 4969-4975). A number of publications describe the ability of this molecule to activate several components of the immune system to kill tumour cells (particularly NK cells and CD8+ T-cells), and develop T-cell memory in order to resist subsequent tumour challenge (Yamane et al. 2009; Expert Opi, Investig. Drugs, 18(7): 991-1000; Neal et al., 2004, Clin. Cancer Res., 1010, 4839-4847).
As IL-2 based immunocytokines can have significant side effects, recent efforts have focused on the reduction of toxicity whilst maintaining efficacy. One example is Selectikine (EMD 521873), which has a substitution of aspartic acid for threonine at position 20 of IL-2, a key residue in the binding of IL-2Rβ (Gillies et al., Clinical Cancer Research, 2011, 17(11), 3673-3685). Selectikine, which binds necrotic tissue, has been shown to have good anti-tumour activity, despite its selectivity for the high affinity IL-2R, over the intermediate IL-2R and good tolerability in Phase I studies (Laurent et al., Journal of Translational Medicine, 2013, 11(1), 5. Available on the World Wide Web at doi.org/10.1186/1479-5876-11-5)
WO2012/178137 (Gillies) describes light chain immunocytokine fusions with tumour targeting antibodies.
An adaptive immune response involves activation, selection, and clonal proliferation of two major classes of lymphocytes termed T-cells and B-cells. After encountering an antigen, T-cells proliferate and differentiate into antigen-specific effector cells, while B-cells proliferate and differentiate into antibody-secreting cells. T-cell activation is a multi-step process requiring several signalling events between the T-cell and an antigen-presenting cell (APC). For T-cell activation to occur, two types of signals must be delivered to a resting T-cell. The first type is mediated by the antigen-specific T-cell receptor (TcR), and confers specificity to the immune response. The second signal, a costimulatory type signal, regulates the magnitude of the response and is delivered through accessory receptors on the T-cell.
A primary costimulatory signal is delivered through the activating CD28 receptor upon engagement of its ligands B7-1 or B7-2. In contrast, engagement of the inhibitory CTLA-4 receptor by the same B7-1 or B7-2 ligands results in attenuation of a T cell response. Thus, CTLA-4 signals antagonize costimulation mediated by CD28. At high antigen concentrations, CD28 costimulation overrides the CTLA-4 inhibitory effect. Temporal regulation of the CD28 and CTLA-4 expression maintains a balance between activating and inhibitory signals and ensures the development of an effective immune response, while safeguarding against the development of autoimmunity.
Programmed death-1 (PD-1) is a 50-55 kDa type I transmembrane receptor that is a member of the CD28 family. PD-1 is involved in the regulation of T-cell activation and is expressed on T cells, B cells, and myeloid cells. Two ligands for PD-1, PD ligand 1 (PD-L1) and ligand 2 (PD-L2) have been identified and have co-stimulatory features.
Programmed cell death 1 ligand 1 (PD-L1), also known as cluster of differentiation (CD274) or B7 homolog 1 (B7-H1), is a member of the B7 family that modulates activation or inhibition of the PD-1 receptor. The open reading frame of PD-L1 encodes a putative type 1 transmembrane protein of 290 amino acids, which includes two extracellular Ig domains (a N-terminal V-like domain and a Ig C-like domain), a hydrophobic transmembrane domain and a cytoplasmic tail of 30 amino acids. The 30 amino acid intracellular (cytoplasmic) domain contains no obvious signalling motifs, but does have a potential site for protein kinase C phosphorylation.
The complete amino acid sequence for PD-L1 can be found in NCBI Reference Sequence: NP_054862.1 (SEQ ID NO: 1), which refers to many journal articles, including, for example, Dong, H., et al. (1999), “PD-L1, a third member of the B7 family, co-stimulates T-cell proliferation and interleukin-10 secretion,” Nat. Med. 5 (12), 1365-1369. The PD-L1 gene is conserved in chimpanzee, Rhesus monkey, dog, cow, mouse, rat, chicken, and zebrafish. The murine form of PD-L1 bears 69% amino acid identity with the human form of PD-L1, and also shares a conserved structure.
In humans, PD-L1 is expressed on a number of immune cell types including activated and anergic/exhausted T-cells, on naive and activated B-cells, as well as on myeloid dendritic cells (DC), monocytes and mast cells. It is also expressed on non-immune cells including islets of the pancreas, Kupffer cells of the liver, vascular endothelium and selected epithelia, for example airway epithelia and renal tubule epithelia, where its expression is enhanced during inflammatory episodes. PD-L1 expression is also found at increased levels on a number of tumours including, but not limited to breast (including but not limited to triple negative breast cancer), ovarian, cervical, colon, colorectal, lung, including non-small cell lung cancer, renal, including renal cell carcinoma, gastric, oesophageal, bladder, hepatocellular cancer, squamous cell carcinoma of the head and neck (SCCHN) and pancreatic cancer, melanoma and uveal melanoma.
PD-1/PD-L1 signalling is believed to serve a critical non-redundant function within the immune system by negatively regulating T cell responses. This regulation is involved in T cell development in the thymus, in regulation of chronic inflammatory responses and in maintenance of both peripheral tolerance and immune privilege. It appears that upregulation of PD-L1 may allow cancers to evade the host immune system and, in many cancers, the expression of PD-L1 is associated with reduced survival and an unfavourable prognosis. Therapeutic monoclonal antibodies that are able to block the PD-1/PD-L1 pathway may enhance anti-tumoural immune responses in patients with cancer. Published clinical data suggest a correlation between clinical responses with tumoural membranous expression of PD-L1 (Brahmer et al., Journal of Clinical Oncology, 2010, Topalian et al., NEJM, 2012) and a stronger correlation between lack of clinical responses and a lack of PD-L1 protein localized to the membrane (Brahmer et al., Journal of Clinical Oncology, 2010, Topalian et al., NEJM, 2012). Thus, PD-L1 expression in tumours or tumour-infiltrating leukocytes (Herbst R S, et al., “Predictive correlates of response to the anti-PD-L1 antibody MPDL3280A in cancer patients”, Nature, 2014, November 27, 515(7528):563-7, doi: 10.1038/nature14011) is a candidate molecular marker for use in selecting patients for immunotherapy, for example, immunotherapy using anti-PD-L1 antibodies. Patient enrichment based on surface expression of PD-L1 may significantly enhance the clinical success of treatment with drugs targeting the PD-1/PD-L1 pathway.
Further evidence of PD-L1 expression and correlation to disease will emerge from the numerous ongoing clinical trials. Atezolizumab is the most advanced, and recent data from Phase II trials shows therapeutic effects in metastatic urothelial carcinoma and NSCLC, particularly in patients with PD-L1+ immune cells in the tumour microenvironment (see Fehrenbacher et al., 2016, The Lancet, http://doi.org/10.1016/S0140-6736(16)00587-0; Rosenberg et al., 2016, The Lancet, http://doi.org/10.1016/S0140-6736(16)00561-4).